1. Field of the Invention
This invention pertains to a system for the actuation of a plurality of actuable devices in situations where it is desirable to prevent the actuation of more than one device upon the occurrence of any one system malfunction or operator error. More specifically, this invention applies to a control system for a nuclear power plant having a working fluid and a load with a multiplicity of parallel load bypass valves and/or atmospheric steam dump valves. In the interest of simplicity, the following discussion will be limited to describing the invention as related to load bypass valves but it should be understood that the invention is equally as applicable to atmospheric dump valves or any plurality of actuable devices. In the operation of a nuclear power plant in which a working fluid is supplied to a load or turbine, the situation occasionally arises where the load or turbine requires a decreased output thereby requiring a decreased supply of energy delivered to it by the working fluid. In such a situation, the excess energy stored in the working fluid that can no longer be utilized by the turbine must be dissipated in order to avoid undesirable consequences such as an energy backup which directly affects the safety of the power plant. One means of dissipating the excess energy is to provide a load bypass system comprising either one large bypass valve or a plurality of smaller bypass valves and/or an atmospheric steam dump valve system. These bypass valve systems provide a substitute load by shunting the working fluid around the load to an energy sink, usually the condenser following the turbine. An essential prerequisite of such a load bypass system is that the bypass valve or valves be prevented from opening erroneously. Utilization of a plurality of bypass valves has the advantage that each of the less-than-full-capacity bypass valves which make up the bypass system can be isolated from the others so that an error or system malfunction can cause the opening of only one valve constituting a small percentage of the total valve capacity. The present invention provides an analogue logic circuit which assures that not more than one valve is open on any one system or operator error.